1. Field of the Invention
The present invention relates to distributed wireless local area networks, and more particularly, to a method for more efficiently transmitting data within a wireless local area network system using a multihop mechanism.
2. Description of the Related Art
The IEEE (802.11) wireless local area network (WLAN) system enables communication between Stations (STAs) and an Access Point (AP) in an infrastructure system or an infrastructure less system (also called Independent BSS or ad hoc network mode). The IEEE 802.11 WLAN system enables single hop communication between STAs in IBSS (Independent Basic Service Set) mode. The access mechanism is a distributed mechanism called Distributed Coordination Function (DCF) and is based on CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance). In addition to physical CS (Carrier Sense), a virtual CS mechanism is used such that a duration value indicates the length of the transmission for each transmitted packet.
Note that a packet may constitute of one or multiple fragment to decrease the risk of packet retransmissions in the case of e.g. interference, where each fragment of a packet is sent after a SIFS (Short Interframe Space) after an acknowledgement from the receiver indicating the successful reception of previous fragment. The duration value sent in a fragment covers the time to transmit the subsequent fragment, if present, plus its corresponding ACK.
Stations receiving the duration value shall not transmit on a wireless media for a period of time equal to the duration value stored in a duration field. In order to handle the so called hidden terminal problem a RTS/CTS mechanism is used.
Presently, multihop support for 802.11 IBSS networks (ad hoc networks) is not available. Multihopping enables stations out of direct reach from each other to communicate through relaying packets via intermediate stations. An additional benefit with multihop support is that by dividing a distance into multiple hops, each hop experiences significantly improved signal reception quality thanks to the power law propagation model. This can be exploited through the usage of a higher link rate that under certain conditions may even reduce the end to end delay.
While the 802.11 protocol does not inherently support multihopping, it does not exclude higher layer protocols with multihop support from being placed on top of existing 802.11 protocols. Currently, the MANET WG in IETF is working on extensions to the TCP/IP protocol suite for mobile ad hoc networks with multihop capabilities. Several MANET protocols such as AODV and DSR have been tested with the 802.11 protocol operating in the IBSS mode.
However, when these routing protocol are used above the 802.11 protocol to provide multihop routing in the ad hoc network without any connection to the radio access protocol, performance problems will arise. For example, when a packet has to travel multiple hops between wireless stations to reach a destination, severe delays may arise due to the nature of the wireless protocol. Collisions can also occur on each link, and the access delays on each hop can add up. In order to achieve a high throughput for TCP transactions perceived by the end user, delay will comprise a vital factor. Thus, enabling control of multihop packets within the 802.11 protocol would greatly enhance overall network performance.